Compact millimeterwave system for De-icing and for preventing the formation of ice on the outer surfaces of shell structures exposed to meterological influences

ABSTRACT

In a millimeter wave de-icing system for the front areas of exposed shell structures, at least one independently operable millimeter wave generator is disposed in each shell structure closely adjacent the surfaces of the shell structure to be de-iced or kept free of ice and uncoupling structures are flanged to the microwave generators and have uncoupling openings disposed along the areas of the shell structure to be heated so as to provide a millimeter wave front directed toward this area. The area subjected to the wavefront includes walls of a dielectric composite material with a metallic skin whereby the millimeter wave front penetrates the wall and at least partially is converted into heat within the wall of composite material thereby providing for rapid and effective heat supply to the wall areas of the shell structure to be kept free of ice.

[0001] This is a Continuation-In-Part application of internationalapplication PCT/EP01/01484 filed Feb. 10, 2001, and claiming thepriority of German application 100 16 259.2 filed Apr. 3, 2000.

BACKGROUND OF THE INVENTION

[0002] The invention relates to a millimeter wave system for thede-icing of front areas of hollow structures structure which are exposedto air flows and, as a result, exposed to meteorological influences andwhich are therefore subject to icing.

[0003] The formation of ice on such structures detrimentally affects theair flow around the structures which results, particularly inaeronautics, to problematic aerodynamic behavior.

[0004] Many efforts have been made to keep the front edges of suchstructures, which are at the greatest risk of icing, free of ice. Theexposed surfaces of the front areas of such surfaces are for examplesprayed or flushed with liquids which prevent the formation of ice, hotair is conducted across the inner surface areas or the areas are heatedelectrically by resistance heating systems. De-icing by liquids islimited by the liquid reservoir required and, furthermore, is consideredto be unreliable.

[0005] It is necessary to suppress the conditions under which ice canform on the respective surfaces. This is possible with liquids only fora limited time, particularly with the use of de-icing liquids on theground before the start. The anti-icing film is torn off already duringthe starting phase and provides during the passage of an airplanethrough cloud formations in which the surfaces are subject to icing,only a relatively small time safety window. Rain washes such ananti-icing film off already on the ground relatively soon.

[0006] In aeronautics, it is common practice to blow hot air taken fromthe engines at the inner surfaces of the wings or, respectively, theaerodynamically important slats, that is the exposed front surfaces,particularly the wing tips. The heat transfer to the slats depends onthe thermodynamic flow conditions and the meteorological circumstancesand also on the travel height, the outside temperature, the travelspeed, the droplet size, the lateral cloud formation, the water contentof the air etc. Taking these parameters into consideration, theefficiency of a hot air anti-icing system is estimated to be about30-40%.

[0007] Such a system results in a high power consumption and also inhigh losses in the supply ducts to the endangered areas of the airplane.In aeronautics, particularly in connection with modern enginetechnology, there are furthermore limits to the removal of sufficientamounts of hot air from the bypass flow of the engines so that it is notalways possible to withdraw a sufficient amount of hot air.

[0008] In another technique, metallic nets or heating mats are disposedin the wall or on the inside wall of such structures which nets or matscan be electrically heated so that, by resistance heating, therespective surface areas can be heated or kept warm as desired. Becauseof the high power requirements, the electric supply lines from theonboard generator to the connecting points of the nets or heating mats,have to have a large cross-section. A homogeneous heating, that isavoiding excessive local heating, particularly in the area of thecontact bars is always problematic when electric power is to be suppliedto an extended area and must be carefully observed. In addition, theheat transfer to the problem areas is generally difficult.

[0009] DE 197 45 621 C1 discloses a de-icing procedure wherein a thinlayer with hydrophobic properties of diamond-like carbon/amorphoushydrocarbon is deposited on the surfaces to be de-iced and, upon theformation of ice, the surface areas are irradiated by an outer infraredradiation source or are heated by a heating mat which is in contact withthe surface areas and are excited and heated thereby.

[0010] DE 197 50 198 C2 discloses a technique for de-icing airplanes bymicrowaves wherein the microwaves are fed to the areas to be de-icedfrom a remote source disposed in the airplane fuselage. Fluiddynamically important areas of an airplane, which are sensitive toicing, consist of compound materials whose dielectric areas arepermeable for microwaves above 20 GHz. For conducting the microwaves,suitable hollow conductors comparable to present hot air pipes, extendin the airplane fuselage within the wings from the microwave generatorup to those areas where the microwaves are then uncoupled and keep theseareas free of ice by heating the dielectric areas. Ice already formed israpidly removed by heating of the interface area of the ice and thesurface on which the ice has formed.

[0011] In lightweight body construction, increasingly hollow body orshell structures including pre-formed, CFK and GFK composite componentsare used. Although such composite materials are very form-stable andrigid and have a high mechanical strength in comparison with metal, theyhave, in comparison with metal, a relatively low an-isotropic thermalconductivity. As a result, heat can build up and the structure mayoverheat whereby local delaminations may occur when they are exposed tohot air. Concerning the flight safety the capability of supplying asufficient power density to the surface area adjacent the air flow,which surface area is potentially coated with ice, is highly limited.

[0012] It is the object of the present invention to provide a compactde-centralized de-icing system for hollow or shell body structures whichare exposed to atmospheric air flow and which are therefore subject tothe formation of ice thereon.

SUMMARY OF THE INVENTION

[0013] In a millimeter wave de-icing system for the front areas ofexposed shell structures, at least one independently operable millimeterwave generator is disposed in each shell structure closely adjacent thesurfaces of the shell structure to be de-iced or kept free of ice anduncoupling structures are flanged to the microwave generators withuncoupling openings disposed along the areas of the shell structure tobe heated so as to provide a millimeter wave front directed toward thisarea. The area subjected to the wave front includes walls of adielectric composite material with a metallic skin whereby themillimeter wave front penetrates the wall and at least partially isconverted into heat within the wall of composite material therebyproviding for rapid and effective heat supply to the wall areas of theshell structure to be kept free of ice.

[0014] To this end, a millimeter wave source whose power output iscontrollable by way of pulse width control is disposed in the interiorof the hollow or shell body structure and an uncoupling arrangement isflanged to the exit of the microwave source directly behind, or as closeas possible behind, the front area which, on the outside, may be subjectto ice formation thereon or which his to be kept free of ice. Themechanically stable hollow or shell body structures consist of CFKmaterials or of GFK materials or of pre-preg compound materials or acomposition thereof. The outer surface of the structure consists of ametal film or a metal skin; at least the aerodynamically exposed outersurface is covered by such a film or skin which is connected along thewhole edge thereof with adjacent metallic structures/surfaces, so thatthese hollow or shell body structures are millimeter wave orhigh-frequency tight and do not permit electromagnetic radiation to beradiated out into the ambient area.

[0015] By way of the uncoupling structure, the millimeter wave radiationis directed onto the front whereby the irradiated compound materialvolume is heated. Within this material, after startup, a temperaturegradient is established which becomes smaller toward the outer skin. Themillimeter wave radiates controllably up to such a power level that, onone hand, at each location of the irradiated compound material volume atemperature-based safety distance of between 35 and 75° C. from thedelaminating temperature of T_(DL)≈130° C. of the compound material canbe maintained and, on the other hand, there is, at the interface withthe metal skin, a thermal surface area power density of up to 46 kW/m²,whereby ice formed on the surface of the structure can be melted so thatit is released from the surface and fully ripped off by the air flow.

[0016] The uncoupling structure of the uncoupling arrangement is ahollow conductor which is flanged to the microwave source or sources andwhich has uncoupling openings for forming the necessary microwave front.They have different sizes and different distances from one another so asto provide for a uniform uncoupling of power along the hollow conductor.The radiation characteristic is such that the phase fronts present alongthe wing contour are as much as possible uniform and their amplitudesprovide the de-icing surface power as locally required. For example, thewing tip may require substantially higher area power densities (up to 60k/m²) than the rearward areas whose requirements may be lower by thefactor 10.

[0017] For the protection of the microwave source, the microwave sourcemay be closed in its transient and blocking attenuation by way ofcirculators.

[0018] Depending on the desired maximum millimeter wave power output,the millimeter wave sources are either same-type klystrons or magnetronsor Extended Interaction Oscillator (EIO's).

[0019] The millimeter wave power exits with little attenuation at theuncoupling structure and selectively heats the surrounding shellstructure, which acts as a low-grade dissipative resonator. Therefore,the hollow, that is, wave conductor with uncoupling structure is made ofan electrically well conducting metal or, if weight savings require it,of compound material which is surrounded by a millimeter wave-tightmetal net. The metal net may also be at the inside of the waveconductor.

[0020] Such a millimeter wave-based de-icing system may be disposed inarrangements or structures of a ship, or a train or a motor vehicle orother hollow structures which have to be kept free of ice and which mayencounter all kinds of meteorological conditions.

[0021] The importance of such a de-icing system in aeronautics withregard to safety is obvious. Airplanes and helicopters have to have anaerodynamically suitable shape, particularly for structures needed forproviding lift and control such as the wings, the rudder, the elevatorsand the edges surrounding the air inlets of the engines.

[0022] In larger airplanes, which, in the lift-relevant area of thefront ends of the wings, have so-called slats, a reliably operatingde-icing system is absolutely necessary for flight safety.

[0023] Another important field of application is the energy generationby wind power plants, which have very large rotor blades and which areconstantly exposed to ground weather conditions. In order to preventicing of the rotor blades, a millimeter wave source is disposed in thehub of the bladed rotor. From there, a hollow conductor with uncouplingstructures extends into each blade and to front areas, which may besubjected to icing.

[0024] The millimeter wave components are arranged directly in the areaof action. Only power supply lines and control lines have to be broughtto the millimeter wave source. There is no need for long hollow guidestructures or wave guides.

[0025] With the millimeter wave de-icing system, flight safety isimproved because of the fast operation of the de- or anti-icing system.During so-called routine anti-icing operation, the power requirementsare relatively low. Furthermore, certain icing conditions can becontrolled which cannot be handled with conventional systems.

[0026] If compound materials are used for the slats, a substantialweight reduction of more than 30% compared to today's metal constructioncan be achieved. Besides the safety, which has first priority, a moreeconomical use of the airplane because of weight reductions and savingsin fuel is possible. In addition, on the ground, the use of de-icingliquids, which are not absolutely safe and detrimentally affect theenvironment, can be reduced or actually eliminated.

[0027] With millimeter wave technology, the temperature of the laminatedstructures can be kept significantly lower than with conventionalde-icing systems blowing hot air at the structures to be de-iced. Thispermits higher power applications per surface area for certain de-icingsituations at the outer skin. In any case, an operation is possible inwhich the compound material and the structure are thermally notstressed. Even clear ice attachment situations, which cannot be handledby the systems existing today, can be controlled without the danger ofoverheating and a resulting delaminating of the compound material.

[0028] For the de-icing of the slats, metal guide tubes in the wings andPiccolo tube systems as they are used today for the distribution of thehot air to the surface areas to be heated are eliminated. This resultsin additional weight savings. Furthermore, the slats can be built asindependent modules, which can be easily exchanged during servicing atthe airport. This is a substantial advantage since it permits timesaving repairs.

[0029] Also, if one of the systems fails, it affects only a single slatsince all the systems are independently operable. Failure of one systemtherefore does not cause substantially deterioration of the airplaneperformance since the individual other slot de-icing systems are notaffected by the failure of the one system and continue to operateindependently (redundancy).

[0030] With the conventional system, upon failure of part of the system,warm air must be withdrawn from the system supplying the other wingwhich substantially reduces the effectiveness of the overall system.

[0031] It is pointed out that all of the electrical power input isconverted 100% into de-icing energy, which is transmitted to themetallic outer skin of the slats.

[0032] Along the slat contour, very different area densities arerequired; the highest requirements are at the front edges of the wingsor slats. In order to prevent so-called run back icing, that is, there-attachment of ice sliding back from the front areas of the slats,also the rear areas of the slats must be heated. In accordance with thenecessary distribution along the slat contour, an optimized hollowmicrowave conductor uncoupling structure with corresponding radiationdistribution characteristics for appropriate area coverage is provided(optimal power adaptation to the slat geometry, see FIG. 4).

[0033] The system is closed with respect to millimeter wave andelectromagnetically sealed. The CFK/composite material is surrounded bya protective metallic skin, which primarily serves as lightningprotection. It also prevents the escape of microwaves from the slatsystem. The closed structure of the slats—hot air requires dischargechannels—has also aerodynamically the advantage that essentially laminarflow conditions can be established at the interface area and disturbingturbulence formation can be prevented.

[0034] The millimeter wave de-icing system can be operated by pulsewidth control so that icing can be avoided in a prophylactic manner fromconditions with small heating requirements up to clear ice removal withthe highest heating power requirements.

[0035] The millimeter wave technical de-icing system operates withoutany losses; the power taken from the net and supplied to the uncouplingstructure is completely converted to de-icing or anti-icing energy. Theperformance of such a system is even more apparent as, with icingalready present, the ice can be released from the exposed surfaceswithin a short time by melting of the interface area.

[0036] The millimeter wave de-icing system which consists of at leastone of the units presented below, will be described in greater detail onthe basis of the accompanying drawings:

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 shows schematically the de-icing arrangement according tothe invention,

[0038]FIG. 2 shows in a cross-sectional view a slat and a front area ofan airplane wing,

[0039]FIG. 3 shows the situation at the front end of a slat of a wing,

[0040]FIG. 4 shows a section of a hollow structure, and

[0041]FIG. 5 shows the temperature distribution in the wall of a hollowstructure.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0042] The millimeter wave de-icing has many applications. Besides theuses on land and on the water, its importance for aeronautics is mostimpressive. It is therefore explained on the basis of the an exemplaryinstallation in the slats of an airplane wing.

[0043]FIG. 1 shows a longitudinal cross-section of one of the slatsindicated in FIG. 4 by the gray structure along the tip of an airplanewing. In the interior of such a slat, the tubular uncouplingstructure/hollow conductor for the microwave is arranged so as to extendparallel to the front edge of the slat (prepping slat) which consists ofCFK material. The resulting wave front is generated by the individualwaves, which have been uncoupled from the hollow conductor by way of theuncoupling openings and are superimposed. Here, the hollow conductor andthe uncoupling structure are the same and mounted directly to themillimeter wave source which comprises a magnetron. The structure isshown in a cross-sectional view in FIG. 2. The de-icing system isdisposed in a chamber of the slat, see top view, FIG. 1.

[0044] A slat, see FIG. 3, forms a closed interior space, which may besubdivided into separate chambers. They may be sealed with regard tomillimeter waves in a simple manner, which is important for an airplanesystem since it should have no influence on the airplane electronicsystem. In the front area of the slat, the heat input should be high; itdecreases in the flow direction, but is present in the required form.Together with the heat generation in the CFK-material itself and by theoperation of the microwave source, the whole slat is maintained at atemperature at which no ice can be formed at the outer surface, not evenin the presence of extremely super-cooled water droplets.

[0045]FIG. 5 shows a mode of operation with the thermal effects thereofon the walls of the slat in the front area thereof. In the figure, for aparticular slat geometry, conventional heating—heat input by blowing hotair at the interior wall—straight line in the figure marked “CFKconventionally heated” is compared with CFK heating by way of millimeterwaves, marked “CFK millimeter wave heated”. The wall of the hollow bodyslat structure is 3 mm thick at the front end of the slat. The wallconsists essentially of CFK material and the thin outer metal skin ofaluminum which is disposed directly on the CFK wall. In FIG. 5, thedelamination temperature of 130° C. of the CFK material is indicated bya dash-dotted horizontal line which highlights the heat problemsencountered with conventional heating in a comparison. During heating ofthe metal skin to 25 to 35° C., hot air of about 110° C. must be blownat the CFK interior wall of the slat in order to generate in thealuminum skin the same temperature as it can be generated by millimeterheating with a temperature at the surface of 80° C. With conventionalheating, the temperature of the CFK material obtained by exposure to aflow of hot air is only 20° C. from the delamination temperature whichis dangerously close. Since, furthermore with hot air heating, ahomogeneous heating cannot be counted on, local delaminations may easilyoccur. With millimeter wave heating, the CFK material remains 50° C.below the dangerous delamination temperature for achieving the samemetal skin temperature, that is, there is a substantially larger safetymargin. Measurements have shown that no hot spots are generated in theCFK structural material.

[0046] Consequently, with the millimeter wave system, areas can bede-iced and can be kept free of ice while the lightweight compoundconstruction materials are not subjected to temperatures which may causea destruction of the structure, not even locally. With the millimeterwave heating, the CFK wall reaches a maximum temperature of slightlyover 80° C. which does not result in thermal stresses of the CFKmaterial at a millimeter wave frequency of ≧20 GHz.

[0047] As shown in the diagram heating by millimeter waves results in ahigher temperature in the wall adjacent the inner surface. The reasonherefor is the volumetric heating effect of the millimeter wave on theCFK volume. It provides for an essentially higher heating efficiency ascompared with conventional heating techniques.

[0048] The significant peak temperature reduction with the use ofmillimeter waves in comparison with conventional heating under otherwiseequal operating and power requirements is achieved in that themillimeter wave penetrates the CFK material and generates heat withinthe first third of the CFK laminate so that there is no need for aheat-conduction-dependent transfer of the heat to the laminate. Fromthere, that is from the location within the wall where the heat isgenerated, the heat flows to the outer skin by conduction. The volume ormaterial heating furthermore permits high heating rates for bringing theouter skin of the slat to the respective melting temperature at therequired area power density. This provides for high dynamics under allpossible conditions.

What is claimed is:
 1. A compact millimeter wave system for preventingicing and de-icing outer surfaces of shell structures exposed tometerological influences and including form-stable hollow structure ofplastic or compound materials with dielectric properties, at least onemillimeter wave source with a controllable power output and a hollowconductor flanged to said millimeter source and including an uncouplingsystem for the release of monochromatic millimeter waves with afrequency ≧20 GHz, said shell structure including a front area which issubject to icing, consisting of a laminated structure including adielectric compound material and a metallic skin disposed on the outsideof said shell structure front area and being in electrical contact withother adjacent electrically conductive components so that a hollow shellspace surrounded by metal is formed, an independently operablemillimeter wave system disposed in each such hollow shell space, saidmillimeter wave system comprising a millimeter wave source with a powersupply, an uncoupling structure connected to said millimeter wave sourceand extending in the interior of said shell structure along the frontarea thereof such that the millimeter waves uncoupled from saiduncoupling structure reach the inner surface of said compound materialin the form of a wavefront and penetrates into the compound materialwhereby the compound material is heated internally and the heat isconducted rapidly to the outer surface of the compound material fromwhere the heat is removed so that the inner surface of the compoundmaterial remains substantially velow the delamination temperature ofabout 130° C. of the compound material while, at the interface betweenthe compound material and the metal skin, a predetermined area energydensity of up to 60 kW/m² may be maintained when the metal skin iscovered by clear ice, whereby the metal skin can be maintained at atemperature of +10° C. to 70° C. depending on the meteorologicalconditions so as to de-ice the metal skin or prevent icing thereof whenthe millimeter wave system is in operation.
 2. A millimeter wave systemaccording to claim 1, wherein said hollow conductor flanged to saidmicrowave source extends along said front area of said shell structureand said uncoupling openings are arranged along said front area anddirected toward said front area such that in millimeter waves uncoupledfrom said hollow conductor through said uncoupling openings superimposeto form said wave front.
 3. A millimeter wave system according to claim2, wherein said millimeter wave source is adapted in its passage andblocking attenuation by way of circulators to the millimeterwave-coupling compound structure representing a consumer.
 4. Amillimeter wave system according to claim 3, wherein said monochromaticmillimeter wave source is, dependent on the power output and frequencyrange desired, one of klystron, magnetron and extended interactionoscillator (EIO).
 5. A millimeter wave system according to claim 4,wherein said hollow conductors and uncoupling structure haveelectrically conductive walls.
 6. A millimeter wave system according toclaim 4, wherein said hollow conductors and uncoupling structuresconsist of one of aluminum and CFK composite material surrounded by anet of electrically well conductive metallic material of a mesh sizesmall enough to prevent the escape of the millimeter wave containedthereby.
 7. A millimeter wave system according to claim 4, wherein sucha system is disposed at the front areas of ships which are sprayed bywater that may form ice under certain atmospheric conditions.
 8. Amillimeter wave system according to claim 4, wherein such a system isdisposed in one of the aerodynamically important structures of an airtransporter which structures need to be kept free of ice.
 9. Amillimeter wave system according to claim 8, wherein said airtransporter is one of an airplane and a helicopter.
 10. A millimeterwave system according to claim 8, wherein said system is disposed in thefront areas of the structures of said air transporter which areimportant for providing lift.
 11. A millimeter wave system according toclaim 10, wherein said system is disposed in the front areas of theelevators and the side rudders of airplanes.
 12. A millimeter wavesystem according to claim 10, wherein said system is disposed in theannular front section around the air inlets of jet engine housings. 13.A millimeter wave system according to claim 9, wherein said system isinstalled in the slats at the front of airplane wings.
 14. A millimeterwave system according to claim 4, wherein said system is installed inrotor blades of wind power generators.